Many electronic and computer-related products such as semiconductors, CD-ROMs, and computer hard disks, require highly polished surfaces in order to achieve optimum operational characteristics. For example, high-quality and extremely precise wafer surfaces are often needed during the production of semiconductor-based integrated circuits. During the fabrication process, the wafers generally undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures created on the wafer surface. As the size of integrated circuits decreases and the density of microstructures on integrated circuits increases, the need for accurate and precise wafer surface polishing increases.
Chemical Mechanical Polishing ("CMP") machines have been developed to polish or planarize semiconductor wafer surfaces to the flat condition desired for integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued Feb. 21, 1989 to Arai et al.; U.S. Pat. No. 4,811,522, issued Mar. 14, 1989 to Gill; U.S. Pat. No. 5,099,614, issued Mar. 31, 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued Jul. 19, 1994 to Karlsrud et al.; U.S. Pat. No. 5,498,196, issued Mar. 12, 1996 to Karlsrud et al.; U.S. Pat. No. 5,498,199, issued Mar. 12, 1996 to Karlsrud et al.; U.S. Pat. No. 5,558,568, issued Sep. 24, 1996 to Talieh et al.; and U.S. Pat. No. 5,584,751, issued Dec. 17, 1996 to Kobayashi et al.
Typically, a CMP machine includes a wafer carrier configured to hold, rotate, and transport a wafer during the process of polishing or planarizing the wafer. The wafer carrier is rotated to cause relative lateral motion between the polishing surface and the wafer to produce a substantially uniform thickness. In general, the polishing surface includes a horizontal polishing pad that has an exposed abrasive surface of cerium oxide, aluminum oxide, fumed/precipitated silica, or other particulate abrasives. Commercially available polishing pads may utilize various materials, as is known in the art. Typically, polishing pads may be formed from a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Ariz. The hardness and density of the polishing pad depends on the material that is to be polished and the degree of precision required in the polishing process.
During a polishing operation, a pressure applying clement (e.g., a rigid plate, a bladder assembly, or the like), which may be integral to the wafer carrier, applies pressure such that the wafer engages the polishing surface with a desired amount of force. The carrier and the polishing pad are rotated, typically at different rotational velocities, to cause relative lateral motion between the polishing pad and the wafer and to promote uniform polishing. Most conventional carrier assemblies include some form of retaining structure that maintains the position of the wafer under the pressure element during polishing. Prior art carrier assemblies designed for compatibility with circular wafers employ round retaining structures such as retaining rings.
Retaining rings may either be fixed or "floating" within the wafer carrier. For example, U.S. Pat. No. 5,695,392, issued Dec. 9, 1997 to Kim, discloses the use of a fixed retaining ring collar that is bolted to the main carrier housing. U.S. Pat. No. 5,584,751, issued Dec. 17, 1996 to Kobayashi et al., and U.S. Pat. No. 5,795,215, issued Aug. 18, 1998 to Guthrie et al., each teach the use of a floating retaining ring and a pressure regulating mechanism that controls the biasing pressure applied to the retaining ring.
Although floating retaining rings may improve the edge profile of the polished wafer (i.e., reduce the amount of tapering or chamfering near the wafer edge due to over polishing), such improvement is typically dependent upon the flatness and precision of the retaining ring itself. For example, if the retaining ring is not completely flat, then it will not compress the polishing pad in a uniform manner. In addition, one-piece retaining rings may roll or tilt during the polishing process (which can also lead to nonuniform compression of the polishing pad). Nonuniform compression of the polishing pad may cause uneven polishing of the wafer, particularly near the wafer edge. Furthermore, polishing of local areas of the wafer edge cannot be controlled with a one-piece retaining ring.
One-piece retaining rings may be difficult to maintain and time consuming to replace. For the reasons discussed above, one-piece retaining rings (whether fixed or floating) may experience uneven wear that can adversely affect the uniformity of the polished wafer. If a one-piece retaining ring has an uneven pressure surface, then it will either need to be replaced or repaired by machining the pressure surface to a desired flatness. The downtime associated with the repair or replacement of a one-piece retaining ring may be extremely undesirable, particularly if the workpiece throughput rate is critical.